In the field of coal pulverizing mills there are generally two types of mills, characterized by the manner in which the pulverized coal is delivered from the mills to a combustion chamber: “suction” mills using exhauster fans to pull the pulverized coal fines from the mill through discharge pipes; and, fanless “pressurized” mills that typically entrain the pulverized coal fines in a stream of pressurized air originating at the mill.
Each type of mill presents its own problems with respect to the goal of supplying an even, balanced flow of coal fines through multiple pipes to multiple burners in the combustion chamber. In suction mills, for example, the exhauster fan tends to throw coal in an unbalanced stream, with heavier particles settling out to one side of the flow through the pipe and lighter fines on the other. In pressurized mills without exhauster fans, distribution problems tend to occur as a result of the varying lengths of discharge pipe leading from the top of the classifier to the various burners around the combustion chamber. Shorter lengths of discharge pipe generally run rich with air (but tend to run lean in coal), while longer lengths of pipe tend to run lean in air (but tend to run rich in coal).
Rich/lean imbalances among the various burners in the combustion chamber produce the usual problems: loss on ignition (LOI) contamination of the ash byproduct; NOX formation; fireball distortion and waterwall erosion; and others known to those skilled in the art.
One common technique for trying to balance coal flow in pipes of different length is known as “clean air flow testing”, in which orifice plate restrictors are placed in the shorter pipes to try to balance air flow with respect to the longer (slower, lower volume) pipes in an air-only test procedure. The problem with clean air flow testing is that, having balanced air flow in a theoretical test, the introduction of coal fines produces fundamentally different results than the air-only testing would indicate, and the orifice plates worsen distribution problems among and within the pipes. As a result, further efforts have attempted on-line adjustable orificing with coal flow present, with similarly disappointing results.
Another approach to balancing coal flow among multiple pipes has been to use a “dynamic” classifier. Dynamic classifiers power-rotate an array of vanes in the classifier cone to decelerate larger particles of coal and encourage lighter fines to travel up and out the classifier into the discharge pipes. It has been found, however, that the use of dynamic classifiers still results in significant differences in distribution among the pipes.
U.S. Pat. No. 6,257,415 and a continuation-in-part thereof (co-pending application Ser. No. 09/901,207) disclose diffuser elements and structures for achieving uniform distribution of coal fines among the individual pipe outlets at the top of a multi-outlet classifier and at multi-outlet branch structures in the network of delivery pipes between the classifier and the combustion chamber; and, a single-pipe diffusion structure for rapid diffusion within the pipe over a short distance. Some of the structures disclosed show a combination of vertical diffuser bars and horizontal diffuser elements, which together diffuse both axial and radial components of uneven flow distributions through a plenum or pipe while minimizing pressure drop.
Installing the above diffuser structure in existing coal delivery pipes can be a difficult job, especially for relatively small diameter single-pipe applications. The pipe sections are welded and/or otherwise sealed to keep the pressurized coal/air flow contained. Securing the diffuser structure to the interior wall surfaces of the pipe requires working in a fairly tight space, often at a distance from the actual point of access to the pipe interior since it is undesirable to open up a pipe section other than at its joint with the next section. The installation becomes more difficult for diffuser structures comprising different types of elements that cooperate with one another in vertically and radially spaced and stacked arrays.
Just as it is desirable to provide equal volumetric balance of coal and air among the burner nozzles directing coal from the pipes into the combustion chamber, it is also important to maintain an even distribution of coal from the exit of each nozzle. Burner nozzles are often provided with internal baffles or “splitter plates” for this purpose.
However, it is common to find sharply-angled turns or elbows in the delivery pipe shortly before the nozzles, the elbows serving to align the outlet end of the pipe with the burner nozzle mounted in the wall of the combustion chamber. Such bends in the pipe often create unevenness in the previously-diffused flow at the critical moment prior to combustion, an unevenness that cannot be fully compensated by splitter plates in the nozzle. One approach to solving this problem has been to place diffuser structure in the pipe between the elbow and the burner nozzle, as shown for example in co-pending application Ser. No. 09/901,207. This typically limits the distance over which diffusion can take place, since the run of pipe from burner to nozzle is usually short, and increases the risk of creating a pressure drop just prior to the burner. Creating a pressure drop at the burner can then adversely affect the previous, upstream attempts at balancing flow through the pipes to the burners. And the placement of diffuser structure in the pipe next to the burner can make it difficult to access the burner through the pipe for frequently needed inspection and repair.